In optical data communication, generally an optical transmitter or transmitting module launches a modulated optical signal into an optical transmission media such as, for example, an optical fiber. The optical signal propagates through the transmission media, experiences various sources of noises and/or waveform distortions, and eventually may be received by an optical receiver. The received optical signal may be subsequently converted into an electrical signal, which carries data information intended for the communication and shall be the same, at least in theory, as that used in modulating the optical signal at the transmitting module.
As is well known in the art, one of the most commonly used modulation scheme in current optical communication is amplitude modulation in a non-return-to-zero (NRZ) format. The NRZ modulation generally has two optical signal levels or signal power levels: a high power level of “1” (or “mark”) and a low power level of “0” (or “non-mark”). An extinction ratio, which may be defined as the power ratio of “1” over “0”, may be used to characterize, among other measures, the quality of a modulated optical signal. Other measures or criteria characterizing a modulated optical signal may include, for example, signal-to-noise ratio (SNR), and waveform distortion characterized typically by, for example, an eye diagram.
As the rate of data transmission increases, the optical signal may suffer more severely from one particular characteristic of the transmission media, that is, dispersion. Dispersion of the optical transmission media, for example fibers including conventional fibers (SMF-28) and dispersion shifted fibers, causes degradation of optical signal quality in ways of distorting the signal waveform, resulting in partially and in some severe situations complete closure of the eye diagram. Although various dispersion compensation techniques have been proposed and in some cases implemented to mitigate the impact of the dispersive transmission media, more needs to be done in order to improve the quality of optical signal transmission, and in particular, optical signal transmission over an extended distance.